As is well-known, the grain oriented silicon steel sheet, wherein secondary recrystallized grains are highly aligned in (110)[001] orientation, namely Goss orientation, is mainly used as a core for transformer and other electrical machinery and equipment. In this case, it is required that the magnetic flux density (represented by B.sub.10 value) is high, the iron loss (represented by W.sub.17/50 value) is low and, in addition to these properties, the magnetostriction property and lamination factor are excellent.
Since these grain oriented silicon steel sheets are usually manufactured through many complicated steps, a great of inventions and improvements are applied to the above steps, whereby low iron loss grain oriented silicon steel sheets having B.sub.10 of not less than 1.90 T and W.sub.17/50 of not more than 1.05 W/kg when the product thickness is 0.30 mm, or B.sub.10 of not less than 1.89 T and W.sub.17/50 of not more than 0.90 W/kg when the product thickness is 0.23 mm are manufactured up to the present.
Lately, supreme demands on the reduction of power loss become considerable in view of energy-saving. Particularly, a system of "Loss Evaluation" wherein the reduction percentage of iron loss is converted into a money to load on the cost of the transformer in the manufacture of low loss transformer is widely spread in Europe and America.
Under the above circumstances, there has recently been proposed a method wherein local microstrain is introduced into the surface of the grain oriented silicon steel sheet by irradiating a laser beam onto the steel sheet surface in a direction substantially perpendicular to the rolling direction after the finish annealing to thereby conduct refinement of magnetic domains and hence reduce the iron loss (Japanese Patent Application Publication Nos. 57-2252, 57-53419, 58-5968, 58-26405, 58-26406, 58-26407 and 58-36051).
Such a magnetic domain refinement is effective for the grain oriented silicon steel sheet not subjected to the strain relief annealing in the manufacture of stacked lamination-core type transformers. However, in case of wound-core type transformers, the strain relief annealing is necessarily performed after the magnetic domain refinement, so that the local microstrain produced by laser irradiation on purpose is released by the annealing treatment to make the width of magnetic domains wide and consequently the laser irradiating effect is lost.
On the other hand, Japanese Patent Application Publication No. 52-24499 discloses a method of producing an extra-low iron loss grain oriented silicon steel sheet wherein the surface of the grain oriented silicon steel sheet is subjected to a mirror finishing after the final annealing or a metal thin plating is applied to the mirror finished surface or further an insulation coating is baked thereon.
However, the mirror finishing for improving the iron loss does not sufficiently contribute to the reduction of iron loss in comparison with remarkable cost-up of the manufacturing step. Particularly, there is a problem on the adhesion property to the insulation coating indispensably applied and baked after the mirror finishing. Therefore, such a mirror finishing is not yet adopted in the present manufacturing step.
Further, there is proposed a method, wherein the steel sheet surface is subjected to the mirror finishing and then a thin coat of oxide ceramics is deposited thereon, in Japanese Patent Application Publication No. 56-4150. In this method, however, the ceramic coat is peeled off from the steel sheet surface when being subjected to a high temperature annealing above 600.degree. C., so that it can not be adopted in the actual manufacturing step.
In addition to the problems accompanied with the countermeasures for the extreme reduction of iron loss as mentioned above, a magnetostriction can not be overlooked in the grain oriented silicon steel sheet. This is a phenomenon that the steel sheet is subjected to stretching vibrations during the magnetization of the steel sheet, which is a most serious cause on the occurrence of noise in the transformer. The magnetostriction behavior results from the fact that the magnetization process of the steel sheet includes 90.degree. boundary displacement and rotation magnetization. That is, the magnetostriction increases in accordance with compressive stress applied to the steel sheet. Since the compressive is irreversibly applied to the steel sheet in the assembling of the transformer, it is advantageous that a tension is previously applied to the steel sheet in view of the compressive stress dependence of magnetostriction. Of course, the application of the tension to the steel sheet is effective for improving the iron loss in the grain oriented silicon steel sheet, and its effect is conspicuous.
In general, the grain oriented silicon steel sheet is subjected to a tension by a double coating consisting of a forsterite layer, which is produced by high temperature reaction between oxides of Si and Fe called as fayalite (Fe.sub.2 SiO.sub.4) usually formed on the steel sheet surface through decarburization and primary recrystallization annealing before secondary recrystallization and an annealing separator composed mainly of MgO in the final annealing, and an insulation coating produced on the forsterite layer (e.g., which consists mainly of phosphate and colloidal silica), whereby the magnetostriction property is improved. However, it can not be said that the compressive stress dependence of magnetostriction is sufficiently improved by such a conventional method.
In order to improve the magnetostriction property, there has been attempted the development of insulation coating capable of applying an elastic tension to the steel sheet surface by high temperature baking of a vitreous insulation coating having a small thermal expansion coefficient (Japanese Patent Application Publication No. 56-521117 or 53-28375), but this attempt is still lacking in the effectiveness.
Further, the lamination factor of the grain oriented silicon steel sheet is expressed by an amount (represented by %) of base metal effectively contributing to magnetic properties, which is obtained by removing forsterite layer and insulation baked coating from the surface of the steel sheet during the final annealing. It is also important to increase this lamination factor. In order to increase the lamination factor, it is known that the surface roughness of the steel sheet is made small or the thickness of each of the forsterite layer and insulation baked coating is made thin. However, although the thinning of these coatings increases the lamination factor, it is very difficult to stably form the thin coating having a good surface appearance and excellent adhesion property and uniformity at the actual manufacturing step, so that there is a limit in the increase of the lamination factor.